Joint arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. Joint arthroplasty is commonly performed for hips, knees, elbows, and other joints. The health and condition of the joint to be replaced dictate the type of prosthesis necessary to replace the natural joint. For example, in a total hip arthroplasty an acetabular cup is implanted in the acetabular cavity in the pelvis to replace the natural acetabulum. Replacement of the acetabulum is necessary when there is an inadequate articulation surface for a head or ball of a prosthetic femoral component.
To implant an acetabular cup, a cavity is reamed in the acetabulum. The acetabular cup is then inserted into the formed cavity and secured by mechanical means, interference fit, or by a combination thereof. The acetabular cup is positioned in the pelvis at a fixed orientation with respect to patient anatomy and should remain stable.
For an acetabular cup relying upon an interference fit alone to secure the prosthesis, it is important that the cavity formed in the acetabulum be sufficiently undersized to provide adequate fixation of the acetabular cup. However, a spherical acetabular cup forcibly inserted into an undersized spherical cavity can, for some patients, result in unacceptable levels of hoop stress.
It is known in some instances that the bone at the cup/acetabulum interface can weaken over time, causing the position of the acetabular cup to shift. Such positional migration of the implanted acetabular cup can erode the surrounding bone of the acetabular cavity. The effect of such bone erosion is the loosening of the acetabular cup, allowing it to further shift in position. The end result of any positional displacement of the acetabular cup can be a non-optimal functioning of the joint or even dislocation of the joint.
Acetabular cup displacement can also generate wear debris, which can interfere with joint articulation. As the acetabular cup moves with respect to the pelvis, the implanted femoral head may no longer articulate in the acetabular cup within a desired range of motion. Improper positioning of the femoral head with respect to the acetabular cup can also accelerate the generation of wear debris creating a need for revision surgery.
Revision procedures in which the acetabular cup is replaced with a new prosthesis pose challenges as the replacement acetabular cup must be securely implanted in the pelvis. Bone erosion that typically necessitates such a revision procedure often produces an acetabular cavity having a shape that does not provide the most favorable conditions for an interference fit with many acetabular cups. Although bone grafts can be effected to reshape the acetabular cavity to be generally spherical, there are certain drawbacks to such bone grafts. For example, a suitable graft bone may not be readily available, it may be difficult to secure the bone graft to the existing bone, and the graft material may not provide sufficient mechanical strength.
Various methods and techniques are presently used to secure an acetabular cup in a cavity formed in the acetabulum of a patient. One such method includes the use of bone cement to secure the acetabular cup to the acetabulum. Another technique utilizes an acetabular cup having holes for receiving screws, or other such fasteners, to affix the acetabular cup to bone.
Acetabular cups with a variety of external geometries are known in the art. For example, U.S. Pat. No. 5,571,201 (Averill et al.) discloses an acetabular cup having a plurality of unidirectional steps for engaging bone. U.S. Pat. No. 5,192,329 (Christie et al.) discloses an oblong revision acetabular cup for implantation into an oblong cavity formed in the acetabulum. U.S. Pat. No. 5,676,704 (Ries et al.) discloses an acetabular cup having an outer surface that gradually thickens in a direction towards the rim of the acetabular cup.
Although, such acetabular cups are able to achieve some degree of fixation in the acetabulum of a patient, an acetabular cup is desired that provides long term fixation and transfers load to the acetabulum anatomically.